Beveled Plane Not Circular
Make sure the bevel factor is set to a value of 0.5.
Also make sure your object's scale is applied in all axis.



Be aware that beveling a 2x2 units plane to a distance of 1 unit will likely create duplicate overlapping vertex at the edge midpoint that may cause problems down the road. It will also turn your four-sided face into a large NGon which is largely regarded as bad practice, and likely to create problems for animation and UV Mapping.